Enforce license banner

[basham]

@jwbennet shared this tip:

We can use a npm tool to automatically enforce license banners on source code. If Conduit starts to grow with more sub-packages or unit tests, this will be a good thing to add to our process.

• https://www.npmjs.com/package/license-check-and-add
• Update license to BSD 3-clause rivet-react#173